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Development of PM Trap System for Urban Buses 960470

In response to stringent particulate matter (PM) emission regulations worldwide, developments of diesel particulate filter (DPF) continue apace in addition to engine modification for PM reduction. Particularly with buses used in urban areas, reduction methods in black smoke emissions are being researched in addition to the efforts to satisfy the aforementioned PM regulations. The system described in this paper was developed for use mainly with buses in large urban concentrations.

The system described in this paper mainly consists of both wall-flow monolith filters for filtration of PM emissions and electric heaters for regeneration. A key feature of this system is that exhaust gas is used for effective combustion of PM during regeneration. With conventional systems, airpumps have been used to feed air for PM combustion during regeneration. With the new system, however, the use of an air pump was discontinued due to durability and cost considerations. Rather, optimization of the exhaust gas flow rate drawn into the filter under regeneration has resulted in regeneration efficiency comparable with those achieved using air pumps. The new system is already in commercial operation with the Yokohama City Transport Bureau.

The difficulty of putting DPF systems to practical use has been due to cracking and melting of filters caused by excessive PM accumulation and subsequent increases in temperature in excess of tolerable levels. To prevent these problems, some means of controlling the accumulated PM mass was required.

Since bus routes in large urban concentrations follow common patterns, the mass of PM accumulated over a given time period is more or less constant. Thus, the amount of PM accumulated over a given time period can be estimated easily and filter damage can be prevented. Nevertheless, application of the new system to buses in cities with different route patterns from those in large urban concentrations was problematic.

To overcome the difficulty of applying the system to buses running on more varied route patterns, a means of sensing the PM accumulation mass with an accuracy of 10% was developed.